Change of State and Latent Energy


During a phase change, the temperature of a substance does not increase or decrease. This is because the thermal energy during a phase change goes into breaking the intermolecular bonds between particles rather than increasing the kinetic energy of the particles. The latent heat is the energy released or absorbed during a change of state. ‘latent’ means hidden or unseen. 


The latent heat is calculated using the formula:

Q=mL 

where:

Q = the heat energy transferred in Joules (J)

m = mass (kg)

L = the latent heat (Jkg−1)


The value of L is dependent on the substance being considered and also the phase change. It takes a lot more energy to change a liquid to a gas than it does a solid to a liquid. This means the latent heat, L, for water to steam will be higher than for ice to water. 

When considering a phase change from solid to liquid (melting), L is described as the latent heat of fusion ({ L }_{ fusion }). When considering a phase change from liquid to a gas (boiling),  L is described as the latent heat of vaporisation ({ L }_{ vaporisation }). 

Consider water: 

  • Latent heat of fusion ({ L }_{ fusion }) = 3.34×105 Jkg−1
  • Latent heat of vaporisation ({ L }_{ vaporisation }) = 22.5×105 Jkg−1

Example 1:

How much energy is required to melt a 5kg block of ice?

Answer:

using: Q=mL 

where:

m = 5 kg

{ L }_{ fusion } = 3.34×105 Jkg−1

Q=5\times (3.34\times { 10 }^{ 5 })

Q=1.67\times { 10 }^{ 6 }J

Q=1670\;kJ

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